The technique disclosed relates to a voltage-driven pixel circuit, a driving method thereof and a display panel.
Organic Electroluminesence Display (OLED) has been increasingly used in the display of high performance as a current-type light emitting device. The traditional Passive Matrix OLED requires shorter time for driving single pixel as the display size increases, and thus requires increasing the instant current which increases the power consumption. At the same time, a large current may lead to a overlarge voltage drop on the ITO line and make the operation voltage of OLED too large, which reduces the efficiency thereof. While Active Matrix OLED (AMOLED) can solve these problems well by scanning input OLED currents progressively using a switching transistor.
In the designing of an AMOLED back plate, however, the unevenness of luminance from pixel to pixel is a problem.
Firstly, AMOLED employs thin film transistors (TFT) to construct a pixel circuit for providing a corresponding current to the OLED device. Low Temperature Poly-silicon thin film transistor (LTPS TFT) or Oxide thin film transistor (Oxide TFT) is often used. The LTPS TFT and Oxide TFT have higher mobility and more stable property and are more suitable for being applied to the display of AMOLED as compared with the general amorphous silicon thin film transistor (a-Si TFT). For the LTPS TFT made on a glass substrate of large area, however, the unevenness in electrical parameters such as a threshold voltage, a mobility or the like arises due to the limitation of a crystallization process and will transform to a current difference or a lightness difference of the OLED display device that will be perceived by human eyes, i.e. a mura phenomena. Although the process evenness of Oxide TFT is better, the threshold voltage thereof will shift under a long-time voltage and high temperature similarly to a-Si TFT, and the shift amounts of the thresholds for the respective parts of the panel will be different due to different display pictures, which results in the difference in display lightness. Since such a difference relates to the image displayed previously, it usually presents as an afterimage phenomena.
Secondly, in a display application of large size, as a power supply line of a back plate has a certain resistance and driving currents for all pixels are supplied by ARVDD, the supply voltage in a region close to a supplying position of the ARVDD power supply in the back plate is higher than that in a region far from the supplying position, which is referred to as IR Drop. Since the voltage of ARVDD is associated with the current, the IR Drop will cause the current differences in different regions, and accordingly a mura occurs in displaying. A LTPS process, which constructs a pixel unit by using P-Type TFTs, is especially sensitive to such a problem, because the storage capacitance thereof is connected between the ARVDD and a gate of TFT and thus a change of the ARVDD voltage will affect the Vgs of the TFT transistor directly.
Thirdly, the unevenness of the film thickness in vapor plating of the OLED device may cause the unevenness of electrical performances. For an a-Si or Oxide TFT process constructing a pixel unit with N-Type TFTs, the storage capacitance thereof is connected between a gate of a driving TFT and an anode of OLED. When a data voltage is transmitted to the gates, the Vgs voltages actually applied on the TFTs will be different if the anode voltages of the respective pixels are different, thereby different driving currents results in different display lightness.
AMOLED can be classified into three types of digital type, current type and voltage type in terms of a driving type. Among these types, a digital type driving method realizes gray levels by using a TFT as a switch for controlling a driving period without compensating for the unevenness. However, an operating frequency thereof increases by folds as the display size increases, leading to a quite large power consumption and reaching a physical limit of the design within a certain range, and thus the digital type driving method is not appropriate for the application of large size. A current type driving method realizes gray levels by supplying currents of different values to driving transistors directly, which may compensate for the unevenness of TFT and for the IR Drop well. However, in case of writing a low gray level signal, charging a relative large parasitic capacitance on a data line with small current results in a too long writing period, which is especially serious and difficult to be addressed in the display of large size. Similar to the traditional AMLCD driving method, a voltage type driving method provides a voltage signal representing the gray level through a driving IC, the voltage signal will be converted into a current signal of the driving transistor inside the pixel circuit and thus the OLED is driven to achieve a gray level for the lightness. Such a method has advantages of fast driving speed, easy for implementation, and thus is suitable for driving panels of large size and is used widely in the field. However, such a method requires designing additional TFTs and capacitance devices for compensating for the unevenness of TFT, the IR Drop and the unevenness of OLED.
FIG. 1 shows a most traditional pixel circuit structure of voltage-driven type formed by two TFT transistors and one capacitance (2T1C). In this structure, a switching transistor T2 transmits a voltage on a data line to a gate of a driving transistor T1, which in turn converts this data voltage into a corresponding current for supplying to an OLED device. In a normal operation, the driving transistor T1 shall be in a saturated region and provides a constant current during a scanning period for one line. The current can be expressed as:
            I      OLED        =                  1        2            ⁢                        μ          n                ·                  C          OX                ·                  W          L                ·                              (                                          V                data                            -                              V                oled                            -                              V                th                                      )                    2                      ,wherein μn is a carrier mobility, COX is a capacitance of gate oxide layer, W/L is a width to length ratio of the transistor, Vdata is the data voltage, Voled is an operating voltage of the OLED shared by all pixel units, and Vth is a threshold voltage of the transistor T1. It can be seen from the above expression, if Vth voltages for respective different pixel units are not the same, the currents thereof differ from each other. If Vth of a pixel shifts as time goes by, it may cause the previous and subsequent currents different, resulting in afterimage. Furthermore, since the unevenness of the OLED devices causes the operating voltages of OLEDs different, it may render the current difference.
There are many pixel structures directed to the unevenness and shift of Vth and the unevenness of the OLED. With respect to a design for a back plate of large size and high resolution, a pixel circuit structure of simple configuration and employing fewer elements is needed.
As for the structure in Reference Document [1] as show in FIG. 2, it can compensate for the unevenness and shift of the Vth of driving transistor T4 only, and fails to compensate for the unevenness of OLED.
As for the structure in Reference Document [2] as shown in FIG. 3, it can compensate for the unevenness and shift of Vth of driving transistor T1 and the unevenness of OLED, but needs a complicated configuration formed by six TFTs and one capacitance.
As for the structure in Reference Document [3] as shown in FIG. 4, it can compensate for the unevenness and shift of driving transistor T1 only, and fails to compensate for the unevenness of OLED.
As for the structure in Reference Document [4] as shown in FIG. 5, it can compensate for the effect from the unevenness and shift of Vth and the unevenness of OLED, but needs 5T2C, for which a design of high opening rate is difficult to be realize.
In a summary, in case of designing an AMOLED pixel structure, the driving circuit can not solve the unevenness of TFT, the IR Drop and the unevenness of OLED very well.
The Reference Documents are as follows.    [1] “A New a-Si:H Thin-Film Transistor Pixel Circuit for Active-Matrix Organic Light-Emitting Diodes” IEEE ELECTRON DEVICE LETTERS, VOL. 24, NO. 9, SEPTEMBER 2003.    [2] “A New a-Si:H TFT Pixel Circuit Compensating the Threshold Voltage Shift of a-Si:H TFT and OLED for Active Matrix OLED” IEEE ELECTRON DEVICE LETTERS, VOL. 26, NO. 12, DECEMBER 2005.    [3] “A New Pixel Circuit for Active Matrix Organic Light Emitting Diodes” IEEE ELECTRON DEVICE LETTERS, VOL. 23, NO. 9, SEPTEMBER 2002.    [4] “Amorphous Oxide TFT Backplane for Large Size AMOLED TVs” SID 2010.